1. Technical Field
The invention relates to a tamper resistant security seal, for example for sealing containers such as security pouches and the like which are used for conveying items of value such as banknotes, letters of credit and the like.
2. Description of the Prior Art
It is extremely important that any such pouches possess tamperproof or tamper-evident seals. Various attempts have been made in the past to make such seals.
Generally these seals are adhesive backed tape constructions which are applied under or over the edge of the pouch flap so that the flap is held in place against the body of the pouch in a firm and tamperproof manner. They can also be in the form of labels.
In recent years the use of metallised films has been replaced by the use of optically embossed metallised films, which have a holographic or diffractive image. Such a construction was recently described by Advanced Holographics in GB-A-2211760. The advantage of using holographic films is that their counterfeiting is extremely difficult. The construction of the general purpose tape is similar to tapes used for hot stamping, for example as described in GB-A-2129739.
Very generally these tapes consist of a supporting film, a wax release layer, and a coating of an embossable thermoplastic polymer which has been subsequently diffractively embossed. Vapour deposited aluminum is then applied with an optional protective layer. The adhesive is then applied from a coating solution. WO-A-88/05728 introduces the general concept of a holographic protective film having a wax interlayer. Then a general purpose pressure sensitive adhesive layer is applied which is in turn protected by a peelable release paper. In use, the tape is fixed to a substrate using the adhesive. It is difficult to copy or alter.
JP-A-63106780 also describes another general purpose holographic tape. The tape is designed with weaker bonding between a protective layer over the holographic layer and an adhesive layer than between a transparent film on which the tape is formed and the holographic layer.
Similar constructions are known for covering large areas of, say, carton card in which an holographic transfer foil is rolled onto the card to produce a card having a diffractive metallic appearance. Such transfer foils are not known to have been used for security pouch seals. However, they are of similar structure to the tapes mentioned above except that instead of a wax release layer the embossable layer is chosen to have release properties from the carrier film.
Searle (GB-B-2136352) discloses holographic seals in which locally embossed areas of thermoplastic polymer are covered by a metallised film which is then demetallised. This leaves areas which are unprotected by the holographic image which is undesirable in case forgery is attempted.
Dai Nippon Insatsu in U.S. Pat. No. 4,856,857 discloses transparent embossed holographic structures in which the holographic impression is supplemented by a partial appearance of the underlying surface which may be a photograph.
Makowka (U.S. Pat. No. 4,834,552) describes making tamper-evident seals for plastic envelopes. The seal is double sided requiring two adhesives and is concealed under the flap in use. Inspection of tamper evidence can only be by folding back the flap and looking at the edges. Paper or cloth having a porous structure is used to protect against low temperature attack.
The use of holographic effects for security purposes is thus well known. The fineness of optical embossing and the nature of the holographic image make it very difficult to alter such devices or manufacture them afresh.
The term "counterfeiting" may be taken to mean the copying of an article by fresh but fraudulent manufacture.
Holographic devices are counterfeit resistant and may be counterfeit indicating. It is relatively difficult to construct an holographic image by "copying" it on a holographic table even if one were available. Slight variations in image quality would also be readily detected in any copy because of the fineness of the surface relief structures employed. The counterfeiter would need to have access to holographic equipment, embossing equipment and metallising equipment to manufacture copies, in practice this would be very difficult.
Holographic seals are also forgery resistant by which is meant alteration resistant. They are also readily alteration indicating, as it is very difficult for a forger to replace accurately any cut away or altered area: the fineness of optical relief embossing acts as a considerable deterrent.
Despite many holographic seal variants disclosed in the art, these all being directed towards enhancing in various ways the anti-counterfeiting properties and/or anti-forgery properties, the importance of providing substitution resistance, which is the third form of attack which a criminal may make, has not hitherto been maximised.
The prior art recognises that holographic seals should not be readily detachable from the substrate to which the seals are attached. Thus for example it is recognised in GB-B-2136352 that the holographic layer should be weak so that attempted removal of the carrier will destroy the holographic embossing.
Similarly in GB-A-2211760 the removal of the carrier film (aided by the strength of the wax it is assumed) will cause damage to the holographic layers.
While such structures have been used previously, they are unlikely to have provided substitution prevention or tamper prevention and possibly tamper indicating properties for example when such substitution or tampering is undertaken at extremes of ambient temperature during freezing or heating. Neither is there any indication in the prior art as to how a superior holographic tape possessing such properties may be made.
By substitution is meant the detachment of all or part of the seal allowing its replacement without giving evidence of that having happened. For example if a seal on a security pouch could be temporarily detached and then resealed without trace, this would be particularly undesirable. Yet many of the prior art seals are susceptible to such action.
By tampering is meant unauthorised interference with the seal whether for the purposes of counterfeiting, forgery or substitution.
In this specification, by printing is meant the application of readable markings of dyes and/or pigments such as those delivered during printing operations, especially thin ink film printing operations such as occur in lithographic, flexographic and gravure printing. The marking may be employed under electronic control such as during laser printing of toners, ink jet printing, thermal transfer printing, impact ribbon printing and the like. Markings may take the form of fine line security indicia, such as alpha numerical characters, symbols, geometrical designs, obliterating coatings and the like.
Markings may also be made caused by printing small shapes which pattern the embossed surface before or after metallisation or by gross embossing number shapes. The printed markings a take the form of single images which may require registration for labels, or the printed markings may take the form of multiple repetitions of a particular design in the form of an endless pattern. Serial or batch numbering may be used to identify individual seals.